The details of the goal, who scored it and how, don’t linger as much in Jeff Agoos’ memory as the impact it had on his team.

For the record, it was Khari Stephenson in the 26th minute. The Jamaican midfielder now plays for the San Jose Earthquakes, but in the fall of 2004 he was going up against his current team as a member of the Kansas City Wizards (now known as Sporting Kansas City), who returned to Arrowhead Stadium for the second leg of their MLS Cup quarterfinal series trailing by two goals on aggregate.

Stephenson’s strike halved San Jose’s advantage with more than an hour left to play.

“That first goal was a backbreaker,” Agoos, the Earthquakes' captain, recalled. “We thought, ‘Let’s get to halftime up two and it would be done.’ Once they scored that goal, the game and the psychology of the game completely changed. We were on our heels the entire time. Then they scored again (early in the second half), and the loss was almost inevitable.”

Kansas City won the game, 3-0, and thus the series and would go on to play in the ’04 MLS Cup final. It marked the second and most recent time an MLS club has overturned a two-goal, first-leg playoff deficit.

The challenge is rare—only nine teams have trailed by at least a pair of goals halfway through a two-game series—and overcoming it is even rarer.

That historic hill is what the Seattle Sounders (down 3-0 to the L.A. Galaxy) and D.C. United (down 3-1 to the Houston Dynamo) face as the MLS Cup semifinals conclude Sunday. Both teams will have a boisterous home crowd in support, but both are battling injuries and will face experienced, playoff-savvy opposition whose only objective is to avoid losing by multiple goals.

Agoos, a Hall of Fame defender and five-time MLS champion, played in both series in which a two-goal cushion evaporated. He won one and lost one and told Sporting News that at this stage of a long season, as high stakes and heavy legs come into play, mindset can be everything.

“In some ways, there’s very little pressure on those teams because they are two goals, and Seattle is three goals, behind,” Agoos said Friday. “It’s important to engage the crowds and get them growing and get momentum because momentum, when you’re down two goals, is everything.

“There’s a lot more psychology in this than there is tactics and technical implications in these games. In both (Seattle and Washington), I think winning that psychological edge for all four teams is going to be most important.”

Playoff soccer isn’t regular-season soccer. That was hammered home when Sounders coach Sigi Schmid said during a brief interview at halftime of last Sunday’s first leg that he’d be fine with a 1-0 loss to the Galaxy.

Normally, defeat is something to avoid. But in a two-game series where goals are the only currency that counts, the permutations are far more numerous and nuanced. A loss can be good enough or a victory insufficient.

On Sunday, Seattle and D.C. must manage 90-minute games differently than they have all season, throwing players into the attack while avoiding sloppiness on defense and considering substitution patterns that might impact the outcome in overtime or a penalty kick shootout. Meanwhile, L.A. and Houston can win by losing—as long as it’s not by too much—which can have a significant effect on tactics and approach.

How each team handles those permutations will determine which two advance to the Dec. 1 MLS Cup final.

“Depending on how sophisticated your team is, you have to be able to adapt and adjust. No game is the same and no opponent is the same,” said Kansas City coach Peter Vermes, whose top-seeded club fell, 2-0, to Houston in the opener of its quarterfinal series and then was eliminated following a 1-0 win in the second leg. “You can’t, at this point in the season, deny who everybody is as a team. It’s not like all of a sudden they can completely change who they are going into the playoffs.”

The Sounders and United must be true to themselves to have a chance, Vermes argued. By playing at home and in the style with which they’re comfortable, both teams “should be able to drive the game,” he said. Then, if and when that first goal goes in, everything can change.

“That puts more pressure on the opposing team. Then, they think about having to score,” Vermes said. “Then, not only the momentum but the mentality can change as well. When you’ve been defending for a long time, now you have to change your way of playing. That’s the difficulty teams face. Just getting that first goal, as soon as you get it, it really does change. It’s game on.”

SKC scored in the 64th minute against Houston and came agonizingly close to the equalizer last week. United (18-11-8), which could take the RFK Stadium field on Sunday (4 p.m. ET, NBC Sports Network) without several key attacking catalysts, likely will need an earlier dose of good fortune. Houston (17-10-11) is built to defend and counter and is near legendary for its ability to grind out postseason results.

A relatively early goal would give D.C. the confidence it needs to stay patient and possess the ball as it hunts for the equalizer. If United plays desperate and leaves holes at the back, the opportunistic Dynamo will capitalize.

“We need to wear this team down, and the way we are going to do that is by possessing the ball,” said D.C. midfielder Nick DeLeon, who has two playoff goals and whose play will be critical in the likely absence of Chris Pontius (groin). “If we can do it over a long period of time, they’re going to get tired chasing us. That is when gaps open up, and that is when we go at them.

“It’s just about being patient. I’m in no rush. Possession will kill that team.”

D.C.’s wild card is Dwayne De Rosario, the 2011 MLS MVP and former Houston hero who should be ready to see his first action since injuring his knee in early September.

“We’ve thought about a lot of different ways to skin this thing,” United coach Ben Olsen told WJFK radio. “Hopefully, we don’t have to make all these crazy moves. Hopefully the natural process, our natural game at home, gets us through. We just need to stick with what we’ve been doing.

“For the first 60 to 70 minutes of this game, we just need to continue playing the way we play at home. ... If it comes down to us needing a goal at the end of the game, then it’s time to throw the kitchen sink and make some crazy adjustments, which means we might be out of our comfort zone.”

Schmid might have to take a look at the plumbing a bit earlier. He’s been here before, of course. Last season, his team faced the only three-goal first-leg deficit in MLS history. The Sounders won the return match over Real Salt Lake, 2-0, but were eliminated. On Sunday at CenturyLink Field, they’ll have to score at least three against the red-hot Galaxy (19-13-6) to force overtime (9 p.m. ET, ESPN).

Injuries will be a factor. Seattle (16-9-12) didn’t pose much of a threat in the first leg absent playmaker Mauro Rosales (questionable with an injured hamstring) and striker Eddie Johnson (likely to play despite a hamstring injury). L.A. could be without defensive midfielder Juninho (Achilles') and Landon Donovan (hamstring), whose presence would lend caution to a Sounders side worried about getting punished on the counter.

“For them, they have confidence. They know how close they were last year to coming from a 3-0 deficit. We know it’s going to be a tough game Sunday,” L.A. defender Sean Franklin said. “We don’t want to go out and obviously open ourselves up. We know they’re going to be throwing numbers up front. It’s all about sticking to our game plan and making sure we’re not opening up big holes for them to get through.”

Seattle’s stadium will be packed, as usual, and tens of thousands of Sounders supporters will be loud and eager for a trip to the club’s first MLS Cup final. Often, such an atmosphere would spell doom for the visitor. But the Galaxy don’t have to face down those fans and score a goal. All they have to do is avoid yielding three. That’s the nature of the playoffs, where soccer’s psychology changes.

Said Galaxy striker Robbie Keane, who has four playoff goals: “It’s a little bit of a game of cat and mouse.”